A boiling water reactor includes a reactor pressure vessel containing a nuclear reactor fuel core submerged in water. The fuel core heats the water to generate steam which is discharged from the reactor pressure vessel through a main steam line and used to power a steam turbine-generator for producing electrical power. Condensate formed in a condenser may be returned to the vessel by a feed pump. Only a small portion (&lt;20%) of the coolant entering the fuel is converted to steam. Thus the fluid exiting the fuel is a mixture of steam and water. The steam/water mixture is routed through a set of steam separators and dryers, the steam being routed to the turbine and the water being mixed with the returned condensate and routed back to the fuel. The flow being returned to the fuel is called "recirculation flow" and is pumped through the fuel by a recirculation pump. The recirculation pump is conventionally installed in a pump housing 2 mounted on the side of the reactor pressure vessel 3, as shown in FIG. 1. The main steam line for steam exiting the reactor pressure vessel on its way to the turbine-generator is indicated by numeral 18.
The reactor pressure vessel is surrounded by a containment vessel 19. The volume inside the containment vessel and outside the reactor pressure vessel forms the drywell 20, which typically contains a noncondensable gas such as nitrogen. The containment vessel 19 is a concrete structure having a steel liner and is designed to withstand elevated pressure inside the drywell.
In accordance with the conventional containment design, an annular suppression pool 22 surrounds the reactor pressure vessel within the containment vessel. The suppression pool is partially filled with water 24 to define a wetwell airspace or plenum 26 thereabove. The suppression pool 22 serves various functions, including being a heat sink in the event of an accident which causes steam from the reactor pressure vessel 3 to leak into the drywell 20. Following the accident, the reactor is shut down, but pressurized steam and residual decay heat continue to be generated for a certain time thereafter. Steam escaping into the drywell 20 is channeled into the suppression pool 22 through vertical downcomer ducts 27 distributed at respective azimuthal positions along the inboard bounding wall 21 of the suppression pool. Each downcomer duct 27 communicates with a plurality of (e.g., three) horizontal vents 28, as seen in FIG. 1.
The circulating pump pressure housing 2 is located in the annular space between the inboard bounding wall 21 of the suppression pool and a circular cylindrical shield wall 1 which encircles the reactor pressure vessel. In the course of reactor maintenance, it may be necessary to replace a defective circulating pump inside pressure housing 2. In that event, the pump motor and impeller assembly must be removed from the housing and a new pump motor and impeller assembly must be installed in its place. To facilitate removal and installation of the pump assembly, the pressure housing 2 has a cover 42 (see FIG. 4) which is removed to provide access to inside the housing. However, the operation of removing and installing a pump motor and impeller assembly must be performed in the narrow annular space between inboard bounding wall 21 of the suppression pool and the shield wall 1. In addition, once the pump assembly has been removed from housing 2, the removed pump assembly must be moved to a specific azimuthal position which has overhead clearance to raise the pump assembly to the level of the refueling floor, where the pump assembly can be transferred to a cart and removed from the drywell. Thus, there is a need for an apparatus which can operate within the confined annular space where the circulating pump is located, and which can remove a pump assembly from or install a pump assembly in the pressure housing with minimum reactor downtime.